Polyvinyl chloride is produced in large quantities throughout the world. The generally used production process is based on the polymerization of vinyl chloride in aqueous suspension or emulsion. In order to achieve optimum product quality the polymerization is conveniently discontinued at a conversion degrees of 90 to 95% of added vinyl chloride. Thus, when the polymerization is finished, significant amounts of unreacted vinyl chloride are present in the reaction vessel (the autoclave). A substantial proportion of said amounts is eliminated by pressure release in the autoclave and recycled to the process. Significant amounts of vinyl chloride, however, remain absorbed in the produced polymer particles. In the further processing of the polymer into powder, in particular during the drying operation, part of said absorbed vinyl chloride escapes, whereas another part remains in the polymer particles and accompanies the product all the way to the manufacturer of the final product where, due to the relatively high processing temperatures being employed, risks are present that the said absorbed residual amounts of vinyl chloride will be set free.
Recently, scientific investigations have shown that vinyl chloride may cause cancer in the liver. Particularly, this applies to persons who for longer time periods have been working in polyvinyl chloride production plants. In most countries, therefore, rather strict requirements have now been set to the working environments in such factories, said requirements being directed to the content of vinyl chloride in the production room and to the amounts of vinyl chloride ventilated to the free air, as well as to the amount of vinyl chloride in the final polymer. These new requirements involve, i.a., that the vinyl chloride content of the finished polymer has to be lowered to a level which was previously regarded as unattainable.
As stated above, a significant proportion of the unreacted vinyl chloride is eliminated and recirculated by pressure release in the reaction vessel itself, and by increasing the temperature during this operation one has already succeeded in eliminating substantially more vinyl chloride than what has previously been the case. However, polyvinyl chloride has a limited heat stability, and by longer residence times at higher temperatures, degradation and, accordingly, a reduction of the product quality will result. Thus, this restricts the possibilities available of increasing the stripping of vinyl chloride at higher temperatures.
It is known to reduce the vinyl chloride content of polyvinyl chloride dispersions by stripping with steam, preferably at reduced pressures in order to reduce the hazard of thermal degradation of the polymer. This operation may be carried out in the polymerization vessel itself, directly after the polymerization has been finished, or in a separate stripping unit. Due to the strict requirements now set to the monomer content of the finished polymer, one must use higher temperatures during the stripping process than the temperature of about 70.degree. C. which has up to now been considered to be an upper limit. By batchwise stripping, one will have a relatively long heating period and cooling period which implies that polymers are subject to an undesirably strong thermal stress. When stripping continuously, it will be of advantage to operate with more stripping units arranged in series because the residence time distribution of the particles can thereby be restricted, and a longer mean residence time can be used without any significant degradation of the polymer.
Instead of stripping units connected in series it has also been proposed to use a multistage stripping column which operates according to the counter-current principle with feeding of the polyvinyl chloride dispersion on the top and steam at the bottom of the column. The stripping column may be designed in different known ways, and columns with filling bodies as well as perforated plate columns may be used.
Both columns present the great disadvantage of polymer particles being deposited. In columns with filling bodies this deposition takes place on and between the filling bodies, whereas in a perforated plate column, the deposition takes place along the walls above the liquid layer on each plate. This deposition occurs in the conventional operation for counter-current perforated plate columns having separate layers of liquid above each plate, because relatively great amounts of steam have to be used, which in turn causes a splashing of dispersion on the wall of the column such that polymer particles remain adhered when the dispersion flows back. Foaming also contributes to such deposition of polymer particles.
Such a deposition of particles causes the operation of the column to be discontinued, or else large amounts of polymer might be degraded by being exposed to excessively high temperatures for longer time periods. The hazard of contamination of the final product is thereby great.
It is therefore an object of this invention to avoid the foregoing disadvantages, particularly the avoidance of the above-discussed deposition of particles and subsequent degradation thereof.